In the last few years, there has been an ever-growing development of high-performance data communications, in terms of both bit rate and quality of service. Some examples are in computer communications, for distributed data processing, for access to distributed information, etc. The introduction of local area network technology represents one of the factors that have made possible this development, and these networks have spread rapidly and continually to the most disparate sectors, often becoming the support for all data communications services within companies. Location of the various offices of the same company in different cities or even different countries gives rise to the problem of interconnecting the various local communications resources.
The commonly adopted solution to this problem entails the realization of private networks based on the use of leased dedicated lines. However, the data traffic generated in the interconnection of local area networks has a typically bursty (or variable-bandwidth) profile; this may lead to a scarcely efficient and flexible use of dedicated resources, so that it may be difficult to amortize the cost involved, which is quite high. A possible alternative could be the use of high bit-rate ATM networks, which are particularly well suited to deal with variable bandwidth traffic and also constitute, for the user, a more economical option than leased lines. In particular, the public operator of the ATM network could provide a broadband virtual private network within the public network.
The paper "Broadband Virtual Private Networks and Their Evolution", presented by S. M. Walters and M. Ahmed at the XIV International Switching Symposium, Yokohama (Japan), 25-30 Oct. 1992, describes a possible structure for a virtual private network, based on the principle of separation between bandwidth allocation and connectivity among "users", indicating with this term not only the actual user terminals but also the various networks to be connected. In other words, it is proposed to realize point-to-point connections between users through virtual channels, without associating any bandwidth characteristic to these channels; bandwidth allocation is performed at the virtual path level. (Note that virtual channels are the lowest multiplexing level in an ATM network and virtual paths are the next higher level). Within a node, virtual channels belonging to different incoming virtual paths must be multiplexed into output virtual paths. Thus the need arises:
a) to apply a statistical multiplexing criterion which allows assigning the bandwidth to output virtual paths in an optimal way; and PA0 b) to guarantee at the same time that the sum of the flows of the different virtual channels constituting the outgoing virtual path never exceeds the bandwidth assigned to it.
In a network exploiting separation between connectivity and bandwidth allocation, the immediate solution for these problems could be:
performing first a cross-connection of the virtual channels, allocating each incoming virtual channel (or all of the virtual channels of the same incoming virtual path that are to be cross-connected to the same outgoing virtual path) its peak bandwidth, which could be that of the virtual path to which it belongs. In general therefore, each outgoing virtual path is allocated a bandwidth equal to the sum of the bandwidths of its contributing virtual paths, i.e. a bandwidth equal to .SIGMA.W.sub.i (i=1,2 . . . m), where m is the number of contributing virtual paths with bandwidth W.sub.i, although on average incoming traffic has a much smaller bandwidth; PA1 policing the traffic egressing from the node in order to assign in the network a peak bandwidth W.sub.u &lt;.SIGMA.W.sub.i to the outgoing virtual path; and PA1 cross-connecting and multiplexing, on high utilization network lines, virtual paths with peak bandwidth W.sub.u. PA1 handle ATM cell flows that have already undergone a first cross-connection phase in the connection network, during which phase the virtual channels of the input virtual paths are distributed among intermediate virtual channel bundles, each having an overall peak bandwidth equal to that of the virtual path of origin of the contributing channels; PA1 cross-connect virtual channels to form shaped and policed virtual paths to be reintroduced into the connection network, where they undergo a second cross-connection phase in order to be transferred to the node output lines; and PA1 carry out traffic shaping at the virtual channel level by selectively discarding cells at the message level, whereby messages are accepted in service queues associated with the output virtual paths only if a length threshold for the respective queue has not been exceeded or, if the threshold has been exceeded, only if the messages have a high priority.
With this solution, however, there is an inefficient allocation of bandwidth resources within the node during the virtual channel cross-connection phase. In fact, with an incoming flow that on average will be equal to the desired outgoing peak bandwidth, it is necessary to allocate a bandwidth equal to .SIGMA.W.sub.i on the output ports of the node, with severe waste of the cross-connect internal resources. To solve this problem, resource-assignment mechanisms collectively known as FRM (Fast Resource Management) have been proposed; however, they have problems or limitations if applied to the case of highly bursty data sources.